Post Sockets, An Abstract Programming Interface for the Transport Layerdraft-trammell-taps-post-sockets-03

TAPS Working Group B. Trammell
Internet-Draft ETH Zurich
Intended status: Informational C. Perkins
Expires: April 30, 2018 University of Glasgow
T. Pauly
Apple Inc.
M. Kuehlewind
ETH Zurich
C. Wood
Apple Inc.
October 27, 2017
Post Sockets, An Abstract Programming Interface for the Transport Layer
draft-trammell-taps-post-sockets-03
Abstract
This document describes Post Sockets, an asynchronous abstract
programming interface for the atomic transmission of messages in an
inherently multipath environment. Post replaces connections with
long-lived associations between endpoints, with the possibility to
cache cryptographic state in order to reduce amortized connection
latency. We present this abstract interface as an illustration of
what is possible with present developments in transport protocols
when freed from the strictures of the current sockets API.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 30, 2018.
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Internet-Draft Post Sockets October 20171. Introduction
The BSD Unix Sockets API's SOCK_STREAM abstraction, by bringing
network sockets into the UNIX programming model, allowing anyone who
knew how to write programs that dealt with sequential-access files to
also write network applications, was a revolution in simplicity. It
would not be an overstatement to say that this simple API is the
reason the Internet won the protocol wars of the 1980s. SOCK_STREAM
is tied to the Transmission Control Protocol (TCP), specified in 1981
[RFC0793]. TCP has scaled remarkably well over the past three and a
half decades, but its total ubiquity has hidden an uncomfortable
fact: the network is not really a file, and stream abstractions are
too simplistic for many modern application programming models.
In the meantime, the nature of Internet access, and the variety of
Internet transport protocols, is evolving. The challenges that new
protocols and access paradigms present to the sockets API and to
programming models based on them inspire the design elements of a new
approach.
Many end-user devices are connected to the Internet via multiple
interfaces, which suggests it is time to promote the paths by which
two endpoints are connected to each other to a first-order object.
While implicit multipath communication is available for these
multihomed nodes in the present Internet architecture with the
Multipath TCP extension (MPTCP) [RFC6824], MPTCP was specifically
designed to hide multipath communication from the application for
purposes of compatibility. Since many multihomed nodes are connected
to the Internet through access paths with widely different properties
with respect to bandwidth, latency and cost, adding explicit path
control to MPTCP's API would be useful in many situations.
Another trend straining the traditional layering of the transport
stack associated with the SOCK_STREAM interface is the widespread
interest in ubiquitous deployment of encryption to guarantee
confidentiality, authenticity, and integrity, in the face of
pervasive surveillance [RFC7258]. Layering the most widely deployed
encryption technology, Transport Layer Security (TLS), strictly atop
TCP (i.e., via a TLS library such as OpenSSL that uses the sockets
API) requires the encryption-layer handshake to happen after the
transport-layer handshake, which increases connection setup latency
on the order of one or two round-trip times, an unacceptable delay
for many applications. Integrating cryptographic state setup and
maintenance into the path abstraction naturally complements efforts
in new protocols (e.g. QUIC [I-D.ietf-quic-transport]) to mitigate
this strict layering.
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To meet these challenges, we present the Post-Sockets Application
Programming Interface (API), described in detail in this work. Post
is designed to be language, transport protocol, and architecture
independent, allowing applications to be written to a common abstract
interface, easily ported among different platforms, and used even in
environments where transport protocol selection may be done
dynamically, as proposed in the IETF's Transport Services working
group.
Post replaces the traditional SOCK_STREAM abstraction with a Message
abstraction, which can be seen as a generalization of the Stream
Control Transmission Protocol's [RFC4960] SOCK_SEQPACKET service.
Messages are sent and received on Carriers, which logically group
Messages for transmission and reception. For backward compatibility,
bidirectional byte stream protocols are represented as a pair of
Messages, one in each direction, that can only be marked complete
when the sending peer has finished transmitting data.
Post replaces the notions of a socket address and connected socket
with an Association with a remote endpoint via set of Paths.
Implementation and wire format for transport protocol(s) implementing
the Post API are explicitly out of scope for this work; these
abstractions need not map directly to implementation-level concepts,
and indeed with various amounts of shimming and glue could be
implemented with varying success atop any sufficiently flexible
transport protocol.
The key features of Post as compared with the existing sockets API
are:
o Explicit Message orientation, with framing and atomicity
guarantees for Message transmission.
o Asynchronous reception, allowing all receiver-side interactions to
be event-driven.
o Explicit support for multistreaming and multipath transport
protocols and network architectures.
o Long-lived Associations, whose lifetimes may not be bound to
underlying transport connections. This allows associations to
cache state and cryptographic key material to enable fast
resumption of communication, and for the implementation of the API
to explicitly take care of connection establishment mechanics such
as connection racing [RFC6555] and peer-to-peer rendezvous
[RFC5245].
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o Transport protocol stack independence, allowing applications to be
written in terms of the semantics best for the application's own
design, separate from the protocol(s) used on the wire to achieve
them. This enables applications written to a single API to make
use of transport protocols in terms of the features they provide,
as in [I-D.ietf-taps-transports].
This work is the synthesis of many years of Internet transport
protocol research and development. It is inspired by concepts from
the Stream Control Transmission Protocol (SCTP) [RFC4960], TCP Minion
[I-D.iyengar-minion-protocol], and MinimaLT [MinimaLT], among other
transport protocol modernization efforts. We present Post as an
illustration of what is possible with present developments in
transport protocols when freed from the strictures of the current
sockets API. While much of the work for building parts of the
protocols needed to implement Post are already ongoing in other IETF
working groups (e.g. MPTCP, QUIC, TLS), we argue that an abstract
programming interface unifying access all these efforts is necessary
to fully exploit their potential.
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may be implemented in any number of ways. The abstract API provides
only for a way for the application to register how it wants to handle
incoming messages.
All the Messages sent to a Carrier will be received on the
corresponding Carrier at the remote endpoint, though not necessarily
reliably or in order, depending on Message properties and the
underlying transport protocol stack.
A Carrier that is backed by current transport protocol stack state
(such as a TCP connection; see Section 2.7) is said to be "active":
messages can be sent and received over it. A Carrier can also be
"dormant": there is long-term state associated with it (via the
underlying Association; see Section 2.3), and it may be able to
reactivated, but messages cannot be sent and received immediately.
Carriers become dormant when the underlying transport protocol stack
determines that an underlying connection has been lost and there is
insufficient state in the Association to re-establish it (e.g., in
the case of a server-side Carrier where the client's address has
changed unexpectedly). Passive close can be handled by the
application via an event on the carrier. Attempting to use a carrier
after passive close results in an error.
If supported by the underlying transport protocol stack, a Carrier
may be forked: creating a new Carrier associated with a new Carrier
at the same remote endpoint. The semantics of the usage of multiple
Carriers based on the same Association are application-specific.
When a Carrier is forked, its corresponding Carrier at the remote
endpoint receives a fork request, which it must accept in order to
fully establish the new carrier. Multiple Carriers between endpoints
are implemented differently by different transport protocol stacks,
either using multiple separate transport-layer connections, or using
multiple streams of multistreaming transport protocols.
To exchange messages with a given remote endpoint, an application may
initiate a Carrier given its remote (see Section 2.4 and local (see
Section 2.5) identities; this is an equivalent to an active open.
There are four special cases of Carriers, as well, supporting
different initiation and interaction patterns, defined in the
subsections below.
o Listener: A Listener is a special case of Message Carrier which
only responds to requests to create a new Carrier from a remote
endpoint, analogous to a server or listening socket in the present
sockets API. Instead of being bound to a specific remote
endpoint, it is bound only to a local identity; however, its
interface for accepting fork requests is identical to that for
fully fledged Carriers.
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o Source: A Source is a special case of Message Carrier over which
messages can only be sent, intended for unidirectional
applications such as multicast transmitters. Sources cannot be
forked, and need not accept forks.
o Sink: A Sink is a special case of Message Carrier over which
messages can only be received, intended for unidirectional
applications such as multicast receivers. Sinks cannot be forked,
and need not accept forks.
o Responder: A Responder is a special case of Message Carrier which
may receive messages from many remote sources, for cases in which
an application will only ever send Messages in reply back to the
source from which a Message was received. This is a common
implementation pattern for servers in client-server applications.
A Responder's receiver gets a Message, as well as a Source to send
replies to. Responders cannot be forked, and need not accept
forks.
2.2. Message
A Message is the unit of communication between applications.
Messages can represent relatively small structures, such as requests
in a request/response protocol such as HTTP; relatively large
structures, such as files of arbitrary size in a filesystem; and
structures of indeterminate length, such as a stream of bytes in a
protocol like TCP.
In the general case, there is no mapping between a Message and
packets sent by the underlying protocol stack on the wire: the
transport protocol may freely segment messages and/or combine
messages into packets. However, a message may be marked as
immediate, which will cause it to be sent in a single packet when
possible.
Content may be sent and received either as Complete or Partial
Messages. Dealing with Complete Messages should be preferred for
simplicity whenever possible based on the underlying protocol. It is
always possible to send Complete Messages, but only protocols that
have a fixed maximum message length may allow clients to receive
Messages using an API that guarantees Complete Messages. Sending and
receiving Partial Messages (that is, a Message whose content spans
multiple calls or callbacks) is always possible.
To send a Message, either Complete or Partial, the Message content is
passed into the Carrier, and client provides a set of callbacks to
know when the Message was delivered or acknowledged. The client of
the API may use the callbacks to pace the sending of Messages.
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To receive a Message, the client of the API schedules a completion to
be called when a Complete or Partial Message is available. If the
client is willing to accept Partial Messages, it can specify the
minimum incomplete Message length it is willing to receive at once,
and the maximum number of bytes it is willing to receive at once. If
the client wants Complete Messages, there are no values to tune. The
scheduling of the receive completion indicates to the Carrier that
there is a desire to receive bytes, effectively creating a "pull
model" in which backpressure may be applied if the client is not
receiving Messages or Partial Messages quickly enough to match the
peer's sending rate. The Carrier may have some minimal buffer of
incoming Messages ready for the client to read to reduce latency.
When receiving a Complete Message, the entire content of the Message
must be delivered at once, and the Message is not delivered at all if
the full Message is not received. This implies that both the sending
and receiving endpoint, whether in the application or the carrier,
must guarantee storage for the full size of a Message.
Partial Messages may be sent or received in several stages, with a
handle representing the total Message being associated with each
portion of the content. Each call to send or receive also indicates
whether or not the Message is now complete. This approach is
necessary whenever the size of the Message does not have a known
bound, or the size is too large to process and hold in memory.
Protocols that only present a concept of byte streams represent their
data as single Messages with unknown bounds. In the case of TCP, the
client will receive a single Message in pieces using the Partial
Message API, and that Message will only be marked as complete when
the peer has sent a FIN.
Messages are sent over and received from Message Carriers (see
Section 2.1).
On sending, Messages have properties that allow the application to
specify its requirements with respect to reliability, ordering,
priority, idempotence, and immediacy; these are described in detail
below. Messages may also have arbitrary properties which provide
additional information to the underlying transport protocol stack on
how they should be handled, in a protocol-specific way. These stacks
may also deliver or set properties on received messages, but in the
general case a received messages contains only a sequence of ordered
bytes. Message properties include:
o Lifetime and Partial Reliability: A Message may have a "lifetime"
- a wall clock duration before which the Message must be available
to the application layer at the remote end. If a lifetime cannot
be met, the Message is discarded as soon as possible. Messages
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without lifetimes are sent reliably if supported by the transport
protocol stack. Lifetimes are also used to prioritize Message
delivery.
There is no guarantee that a Message will not be delivered after
the end of its lifetime; for example, a Message delivered over a
strictly reliable transport will be delivered regardless of its
lifetime. Depending on the transport protocol stack used to
transmit the message, these lifetimes may also be signalled to
path elements by the underlying transport, so that path elements
that realize a lifetime cannot be met can discard frames
containing the Messages instead of forwarding them.
o Priority: Messages have a "niceness" - a priority among other
messages sent over the same Carrier in an unbounded hierarchy most
naturally represented as a non-negative integer. By default,
Messages are in niceness class 0, or highest priority. Niceness
class 1 Messages will yield to niceness class 0 Messages sent over
the same Carrier, class 2 to class 1, and so on. Niceness may be
translated to a priority signal for exposure to path elements
(e.g. DSCP code point) to allow prioritization along the path as
well as at the sender and receiver. This inversion of normal
schemes for expressing priority has a convenient property:
priority increases as both niceness and lifetime decrease. A
Message may have both a niceness and a lifetime - Messages with
higher niceness classes will yield to lower classes if resource
constraints mean only one can meet the lifetime.
o Dependence: A Message may have "antecedents" - other Messages on
which it depends, which must be delivered before it (the
"successor") is delivered. The sending transport uses deadlines,
niceness, and antecedents, along with information about the
properties of the Paths available, to determine when to send which
Message down which Path.
o Idempotence: A sending application may mark a Message as
"idempotent" to signal to the underlying transport protocol stack
that its application semantics make it safe to send in situations
that may cause it to be received more than once (i.e., for 0-RTT
session resumption as in TCP Fast Open, TLS 1.3, and QUIC).
o Immediacy: A sending application may mark a Message as "immediate"
to signal to the underlying transport protocol stack that its
application semantics require it to be placed in a single packet,
on its own, instead of waiting to be combined with other messages
or parts thereof (i.e., for media transports and interactive
sessions with small messages).
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Senders may also be asynchronously notified of three events on
Messages they have sent: that the Message has been transmitted, that
the Message has been acknowledged by the receiver, or that the
Message has expired before transmission/acknowledgement. Not all
transport protocol stacks will support all of these events.
2.3. Association
An Association contains the long-term state necessary to support
communications between a Local (see Section 2.5) and a Remote (see
Section 2.4) endpoint, such as trust model information, including
pinned public keys or anchor certificates, cryptographic session
resumption parameters, or rendezvous information. It uses
information from the Configuration (see Section 2.6) to constrain the
selection of transport protocols and local interfaces to create
Transients (see Section 2.7) to carry Messages; and information about
the paths through the network available available between them (see
Section 2.8).
All Carriers are bound to an Association. New Carriers will reuse an
Association if they can be carried from the same Local to the same
Remote over the same Paths; this re-use of an Association may implies
the creation of a new Transient.
Associations may exist and be created without a Carrier. This may be
done if peer cryptographic state such as a pre-shared key is
established out-of-band. Thus, Associations may be created without
the need to send application data to a peer, that is, without a
Carrier. Associations are mutable. Association state may expire
over time, after which it is removed from the Association, and
Transients may export cryptographic state to store in an Association
as needed. Moreover, this state may be exported directly into the
Association or modified before insertion. This may be needed to
diversify ephemeral Transient keying material from the longer-term
Association keying material.
A primary use of Association state is to allow new Associations and
their derived Carriers to be quickly created without performing in-
band cryptographic handshakes. See [I-D.kuehlewind-taps-crypto-sep]
for more details about this separation.
2.4. Remote
A Remote represents information required to establish and maintain a
connection with the far end of an Association: name(s), address(es),
and transport protocol parameters that can be used to establish a
Transient; transport protocols to use; trust model information,
inherited from the relevant Association, used to identify the remote
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on connection establishment; and so on. Each Association is
associated with a single Remote, either explicitly by the application
(when created by the initiation of a Carrier) or a Listener (when
created by forking a Carrier on passive open).
A Remote may be resolved, which results in zero or more Remotes with
more specific information. For example, an application may want to
establish a connection to a website identified by a URL
https://www.example.com. This URL would be wrapped in a Remote and
passed to a call to initiate a Carrier. The first pass resolution
might parse the URL, decomposing it into a name, a transport port,
and a transport protocol to try connecting with. A second pass
resolution would then look up network-layer addresses associated with
that name through DNS, and store any certificates available from
DANE. Once a Remote has been resolved to the point that a transport
protocol stack can use it to create a Transient, it is considered
fully resolved.
2.5. Local
A Local represents all the information about the local endpoint
necessary to establish an Association or a Listener. It encapsulates
the Provisioning Domain (PvD) of a single interface in the multiple
provisioning domain architecture [RFC7556], and adds information
about the service endpoint (transport protocol port), and, per
[I-D.pauly-taps-transport-security], cryptographic identities
(certificates and associated private keys) bound to this endpoint.
2.6. Configuration
A Configuration encapsulates an application's preferences around Path
selection and protocol options.
Each Association has exactly one Configuration, and all Carriers
belonging to that Association share the same Configuration.
The application cannot modify the Configuration for a Carrier or
Association once it is set. If a new set of options needs to be
used, then the application needs a new Carrier or Association
instance. This is necessary to ensure that a single Carrier can
consistently track the Paths and protocol options it uses, since it
is usually not possible to modify these properties without breaking
connectivity.
To influence Path selection, the application can configure a set of
requirements, preferences, and restrictions concerning which Paths
may be selected by the Association to use for creating Transients
between a Local and a Remote. For example, a Configuration can
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specify that the application prefers Wi-Fi access over LTE when
roaming on a foreign LTE network, due to monetary cost to the user.
The Association uses the Configuration's Path preferences as a key
part of determining the Paths to use for its Transients. The
Configuration is provided as input when examining the complete list
of available Paths on the system (to limit the list, or order the
Paths by preference). The system's policy will further restrict and
modify the Path that is ultimately selected, using other aspects of
the Configuration (protocol options and originating application) to
select the most appropriate Path.
To influence protocol selection and options, the Configuration
contains one or more allowed Protocol Stack Configurations. Each of
these is comprised of application- and transport-layer protocols that
may be used together to communicate to the Remote, along with any
protocol-specific options. For example, a Configuration could
specify two alternate, but equivalent, protocol stacks: one using
HTTP/2 over TLS over TCP, and the other using QUIC over UDP.
Alternatively, the Configuration could specify two protocol stacks
with the same protocols, but different protocol options: one using
TLS with TLS 1.3 0-RTT enabled and TCP with TCP Fast-Open enabled,
and one using TLS with out 0-RTT and TCP without TCP Fast-Open.
Protocol-specific options within the Configuration include trust
settings and acceptable cryptographic algorithms to be used by
security protocols. These may be configured for specific protocols
to allow different settings for each (such as between TLS over TCP
and TLS for use with QUIC), or set as default security settings on
the Configuration to be used by any protocol that needs to evaluate
trust. Trust settings may include certificate anchors and
certificate pinning options.
2.7. Transient
A Transient represents a binding between a Carrier and the instance
of the transport protocol stack that implements it. As an
Association contains long-term state for communications between two
endpoints, a Transient contains ephemeral state for a single
transport protocol over a one or more Paths at a given point in time.
A Carrier may be served by multiple Transients at once, e.g. when
implementing multipath communication such that the separate paths are
exposed to the API by the underlying transport protocol stack. Each
Transient serves only one Carrier, although multiple Transients may
share the same underlying protocol stack; e.g. when multiplexing
Carriers over streams in a multistreaming protocol.
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Transients are generally not exposed by the API to the application,
though they may be accessible for debugging and logging purposes.
2.8. Path
A Path represents information about a single path through the network
used by an Association, in terms of source and destination network
and transport layer addresses within an addressing context, and the
provisioning domain [RFC7556] of the local interface. This
information may be learned through a resolution, discovery, or
rendezvous process (e.g. DNS, ICE), by measurements taken by the
transport protocol stack, or by some other path information discovery
mechanism. It is used by the transport protocol stack to maintain
and/or (re-)establish communications for the Association.
The set of available properties is a function of the transport
protocol stacks in use by an association. However, the following
core properties are generally useful for applications and transport
layer protocols to choose among paths for specific Messages:
o Maximum Transmission Unit (MTU): the maximum size of an Message's
payload (subtracting transport, network, and link layer overhead)
which will likely fit into a single frame. Derived from signals
sent by path elements, where available, and/or path MTU discovery
processes run by the transport layer.
o Latency Expectation: expected one-way delay along the Path.
Generally provided by inline measurements performed by the
transport layer, as opposed to signaled by path elements.
o Loss Probability Expectation: expected probability of a loss of
any given single frame along the Path. Generally provided by
inline measurements performed by the transport layer, as opposed
to signaled by path elements.
o Available Data Rate Expectation: expected maximum data rate along
the Path. May be derived from passive measurements by the
transport layer, or from signals from path elements.
o Reserved Data Rate: Committed, reserved data rate for the given
Association along the Path. Requires a bandwidth reservation
service in the underlying transport protocol stack.
o Path Element Membership: Identifiers for some or all nodes along
the path, depending on the capabilities of the underlying network
layer protocol to provide this.
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Path properties are generally read-only. MTU is a property of the
underlying link-layer technology on each link in the path; latency,
loss, and rate expectations are dynamic properties of the network
configuration and network traffic conditions; path element membership
is a function of network topology. In an explicitly multipath
architecture, application and transport layer requirements can be met
by having multiple paths with different properties to select from.
Transport protocol stacks can also provide signaling to devices along
the path, but this signaling is derived from information provided to
the Message abstraction.
3. Abstract Programming Interface
We now turn to the design of an abstract programming interface to
provide a simple interface to Post's abstractions, constrained by the
following design principles:
o Flexibility is paramount. So is simplicity. Applications must be
given as many controls and as much information as they may need,
but they must be able to ignore controls and information
irrelevant to their operation. This implies that the "default"
interface must be no more complicated than BSD sockets, and must
do something reasonable.
o Reception is an inherently asynchronous activity. While the API
is designed to be as platform-independent as possible, one key
insight it is based on is that an Message receiver's behavior in a
packet-switched network is inherently asynchronous, driven by the
receipt of packets, and that this asynchronicity must be reflected
in the API. The actual implementation of receive and event
handling will need to be aligned to the method a given platform
provides for asynchronous I/O.
o A new API cannot be bound to a single transport protocol and
expect wide deployment. As the API is transport-independent and
may support runtime transport selection, it must impose the
minimum possible set of constraints on its underlying transports,
though some API features may require underlying transport features
to work optimally. It must be possible to implement Post over
vanilla TCP in the present Internet architecture.
The API we design from these principles is centered around a Carrier,
which can be created actively via initiate() or passively via a
listen(); the latter creates a Listener from which new Carriers can
be accept()ed. Messages may be created explicitly and passed to this
Carrier, or implicitly through a simplified interface which uses
default message properties (reliable transport without priority or
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deadline, which guarantees ordered delivery over a single Carrier
when the underlying transport protocol stack supports it).
For each connection between a Local and a Remote a new Carrier is
created and destroyed when the connection is closed. However, a new
Carrier may use an existing Association if present for the requested
Local-Remote pair and permitted by the PolicyContext that can be
provided at Carrier initiation. Further the system-wide
PolicyContext can contain more information that determine when to
create or destroy Associations other than at Carrier initiation.
E.g. an Association can be created at system start, based on the
configured PolicyContext or also by a manual action of an single
application, for Local-Remote pairs that are known to be likely used
soon, and to pre-establish, e.g., cryptographic context as well as
potentially collect current information about path capabilities.
Every time an actual connection with a specific PSI is established
between the Local and Remote, the Association learns new Path
information and stores them. This information can be used when a new
transient is created, e.g. to decide which PSI to use (to provide the
highest probably for a successful connection attempt) or which PSIs
to probe for (first). A Transient is created when an application
actually sends a Message over a Carrier. As further explained below
this step can actually create multiple transients for probing or
assign a new transient to an already active PSI, e.g. if multi-
streaming is provided and supported for these kind of use on both
sides.
3.1. Example Connection Patterns
Here, we illustrate the usage of the API for common connection
patterns. Note that error handling is ignored in these illustrations
for ease of reading.
3.1.1. Client-Server
Here's an example client-server application. The server echoes
messages. The client sends a message and prints what it receives.
The client in Figure 2 connects, sends a message, and sets up a
receiver to print messages received in response. The carrier is
inactive after the Initiate() call; the Send() call blocks until the
carrier can be activated.
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The fundamental design of a client need not change at all for happy
eyeballs [RFC6555] (selection of multiple potential protocol stacks
through connection racing); this is handled by the Post Sockets
implementation automatically. If this connection racing is to use
0-RTT data (i.e., as provided by TCP Fast Open [RFC7413], the client
must mark the outgoing message as idempotent.
// connect to a server given a remote and send some 0-RTT data
func sayHelloQuickly() {
carrier := Initiate(local, remote)
carrier.SendMsg(OutMessage{Content: []byte("Hello!"), Idempotent: true}, nil, nil, nil)
carrier.Ready(func (msg InMessage) {
fmt.Println(string([]byte(msg)))
return false
})
carrier.Close()
}
3.1.3. Peer to Peer with Network Address Translation
In the client-server examples shown above, the Remote given to the
Initiate call refers to the name and port of the server to connect
to. This need not be the case, however; a Remote may also refer to
an identity and a rendezvous point for rendezvous as in ICE
[RFC5245]. Here, each peer does its own Initiate call
simultaneously, and the result on each side is a Carrier attached to
an appropriate Association.
3.1.4. Multicast Receiver
A multicast receiver is implemented using a Sink attached to a Local
encapsulating a multicast address on which to receive multicast
datagrams. The following example prints messages received on the
multicast address forever.
func receiveMulticast() {
sink = NewSink(local)
sink.Ready(func (msg InMessage) {
fmt.Println(string([]byte(msg)))
return true
})
}
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Internet-Draft Post Sockets October 20173.1.5. Association Bootstrapping
Here, we show how Association state may be initialized without a
carrier. The goal is to create a long-term Association from which
Carriers may be derived and, if possible, used immediately. Per
[I-D.pauly-taps-transport-security], a first step is to specify trust
model constraints, such as pinned public keys and anchor
certificates, which are needed to create Remote connections.
We begin by creating shared security parameters that will be used
later for creating a remote connection.
// create security parameters with a set of trusted certificates
func createParameters(trustedCerts []Certificate) Parameters {
parameters := Parameters()
parameters = parameters.SetTrustedCerts(trustedCerts)
return parameters
}
Using these statically configured parameters, we now show how to
create an Association between a Local and Remote using these
parameters.
// create an Association using shared parameters
func createAssociation(local Local, remote Remote, parameters Parameters) Association {
association := NewAssociation(local, remote, parameters)
return association
}
We may also create an Association with a pre-shared key configured
out-of-band.
// create an Association using a pre-shared key
func createAssociationWithPSK(local Local, remote Remote, parameters Parameters, preSharedKey []byte) Association {
association := NewAssociation(local, remote, parameters)
association = association.SetPreSharedKey(preSharedKey)
return association
}
We now show how to create a Carrier from an existing, pre-configured
Association. This Association may or may not contain shared
cryptographic static between the Local and Remote, depending on how
it was configured.
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// open a connection to a server using an existing Association and send some data,
// which will be sent early if possible.
func sayHelloWithAssociation(association Association) {
carrier := association.Initiate()
carrier.SendMsg(OutMessage{Content: []byte("Hello!"), Idempotent: true}, nil, nil, nil)
carrier.Ready(func (msg InMessage) {
fmt.Println(string([]byte(msg)))
return false
})
carrier.Close()
}
3.2. API Dynamics
As Carriers provide the central entry point to Post, they are key to
API dynamics. The lifecycle of a carrier is shown in Figure 5.
Carriers are created by active openers by calling Initiate() given a
Local and a Remote, and by passive openers by calling Listen() given
a Local; the .Accept() method on the listener Carrier can then be
used to create active carriers. By default, the underlying
Association is automatically created and managed by the underlying
API. This underlying Association can be accessed by the Carrier's
.Association() method. Alternately, an association can be explicitly
created using NewAssociation(), and a Carrier on the association may
be accessed or initiated by calling the association's .Initiate()
method.
Once a Carrier has been created (via Initiate(),
Association.Initiate(), NewSource(), NewSink(), or
Listen()/Accept()), it may be used to send and receive Messages. The
existence of a Carrier does not imply the existence of an active
Transient or associated transport-layer connection; these may be
created when the carrier is, or may be deferred, depending on the
network environment, configuration, and protocol stacks available.
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Listen(local) Initiate(local,remote) NewSource(local,remote)
| | or NewSink(local)
[ Carrier ] | |
[(listener)] +--------------------+
| V
.Accept()-----------> [Carrier] -+----------> .Close()
| ^ | close [ Carrier ]
| | +- event -> [ (closed) ]
| |
.Association() .Carriers()
| .Initiate()
V |
[Association]
^
|
NewAssociation(local,remote)
Figure 5: Carrier and Association Life Cycle
Access to more detailed information is possible through accessors on
Carriers and Associations, as shown in Figure 6. The set of
currently active Transients can be accessed through the Carrier's
.Transients() methods. The active path(s) used by a Transient can be
accessed through the Transient's .Paths() method, and the set of all
paths for which properties are cached by an Association can be
accessed through the Association's .Paths() method. The set of
active carriers on an association can be accessed through the
Association's .Carriers() method. Access to transients and paths is
not necessary in normal operation; these accessors are provided
primarily for logging and debugging purposes.
[Carrier]---.Transients()--->[Transient]
| ^ |
| | |
.Association() .Carriers() .Paths()
| .Initiate() |
V | V
[Association]---.Paths()------>[Path]
Figure 6: Accessors on Carriers and Associations
Each Carrier has a .Send() method, by which Messages can be sent with
given properties, and a .Ready() method, which supplies a callback
for reading Messages from the remote side. .Send() is not available
on Sinks, and .Ready() is not available on Sources. Carriers also
provide .OnSent(), .OnAcked(), and .OnExpired() calls for binding
default send event handlers to the Carrier, and .OnClosed() for
handling passive close notifications.
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+---------[incoming]-----------+
| [Message ] V
[outgoing] ---> .Send() ---> [Carrier] <---- .Ready() <---- [Receiver]
[Message ] |
+--- .OnSent()
+--- .OnAcked()
+--- .OnExpired()
+--- .OnClosed()
Figure 7: Sending and Receiving Messages and Events
An application may have a global Configuation, as well as more
specific Configurations to apply to the establishment of a given
Association or Carrier. These Configurations are optional arguments
to the Association and Carrier creation calls.
In order to initiate a connection with a remote endpoint, a user of
Post Sockets must start from a Remote (see Section 2.4). A Remote
encapsulates identifying information about a remote endpoint at a
specific level of resolution. A new Remote can be wrapped around
some identifying information by via the NewRemote() call. A Remote
has a .Resolve() method, which can be iteratively revoked to increase
the level of resolution; a call to Resolve on a given Remote may
result in one to many Remotes, as shown in Figure 8. Remotes at any
level of resolution may be passed to Post Sockets calls; each call
will continue resolution to the point necessary to establish or
resume a Carrier.
+----------------------------+
n | | 1
NewRemote(identifiers) ---+--->[Remote] --.Resolve()---+
Figure 8: Recursive resolution of Remotes
Information about the local endpoint is also necessary to establish
an Association, whether explicitly or implicitly through the creation
of a Carrier or Listener. This is passed in the form of a Local (see
Section 2.5). A Local is created with a NewLocal() call, which takes
a Configuration (including certificates to present and secret keys
associated with them) and identifying information (interface(s) and
port(s) to use).
4. Implementation Considerations
Here we discuss an incomplete list of API implementation
considerations that have arisen with experimentation with prototype
implementations of Post.
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Internet-Draft Post Sockets October 20174.1. Protocol Stack Instance (PSI)
A PSI encapsulates an arbitrary stack of protocols (e.g., TCP over
IPv6, SCTP over DTLS over UDP over IPv4). PSIs provide the bridge
between the interface (Carrier) plus the current state (Transients)
and the implementation of a given set of transport services
[I-D.ietf-taps-transports].
A given implementation makes one or more possible protocol stacks
available to its applications. Selection and configuration among
multiple PSIs is based on system-level or application policies, as
well as on network conditions in the provisioning domain in which a
connection is made.
+=========+ +=========+ +==========+ +==========+
| Carrier | | Carrier | | Carrier | | Carrier |
+=========+ +=========+ +==========+ +==========+
| | | |
+=========+ +=========+ +==========+ +==========+
|Transient| |Transient| |Transient | |Transient |
+=========+ +=========+ +==========+ +==========+
| \ / / \
+=========+ +=========+ +=========+ +=========+
| PSI | | PSI | | PSI | | PSI |
+===+-----++ +===+-----++ +===+-----++ ++-----+===+
|TLS | |SCTP | |TLS | | TLS|
|TCP | |DTLS | |TCP | | TCP|
|IPv6 | |UDP | |IPv6 | | IPv4|
|802.3 | |IPv6 | |802.11| |802.11|
+------+ |802.3 | +------+ +------+
+------+
(a) Transient (b) Carrier multiplexing (c) Multiple candidates
bound to PSI over a multi-streaming racing during session
transport protocol establishment
Figure 9: Example Protocol Stack Instances
For example, Figure 9(a) shows a TLS over TCP stack, usable on most
network connections. Protocols are layered to ensure that the PSI
provides all the transport services required by the application. A
single PSI may be bound to multiple Carriers, as shown in
Figure 9(b): a multi-streaming transport protocol like QUIC or SCTP
can support one carrier per stream. Where multi-streaming transport
is not available, these carriers could be serviced by different PSIs
on different flows. On the other hand, multiple PSIs are bound to a
single transient during establishment, as shown in Figure 9(c).
Here, the losing PSI in a happy-eyeballs race will be terminated, and
the carrier will continue using the winning PSI.
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Internet-Draft Post Sockets October 20174.2. Message Framing, Parsing, and Serialization
While some transports expose a byte stream abstraction, most higher
level protocols impose some structure onto that byte stream. That
is, the higher level protocol operates in terms of messages, protocol
data units (PDUs), rather than using unstructured sequences of bytes,
with each message being processed in turn. Protocols are specified
in terms of state machines acting on semantic messages, with parsing
the byte stream into messages being a necessary annoyance, rather
than a semantic concern. Accordingly, Post Sockets exposes a
message-based API to applications as the primary abstraction.
Protocols that deal only in byte streams, such as TCP, represent
their data in each direction as a single, long message. When framing
protocols are placed on top of byte streams, the messages used in the
API represent the framed messages within the stream.
There are other benefits of providing a message-oriented API beyond
framing PDUs that Post Sockets should provide when supported by the
underlying transport. These include:
o the ability to associate deadlines with messages, for transports
that care about timing;
o the ability to provide control of reliability, choosing what
messages to retransmit in the event of packet loss, and how best
to make use of the data that arrived;
o the ability to manage dependencies between messages, when some
messages may not be delivered due to either packet loss or missing
a deadline, in particular the ability to avoid (re-)sending data
that relies on a previous transmission that was never received.
All require explicit message boundaries, and application-level
framing of messages, to be effective. Once a message is passed to
Post Sockets, it can not be cancelled or paused, but prioritization
as well as lifetime and retransmission management will provide the
protocol stack with all needed information to send the messages as
quickly as possible without blocking transmission unnecessarily.
Post Sockets provides this by handling message, with known identity
(sequence numbers, in the simple case), lifetimes, niceness, and
antecedents.
Transport protocols such as SCTP provide a message-oriented API that
has similar features to those we describe. Other transports, such as
TCP, do not. To support a message oriented API, while still being
compatible with stream-based transport protocols, Post Sockets must
provide APIs for parsing and serialising messages that understand the
protocol data. That is, we push message parsing and serialisation
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down into the Post Sockets stack, allowing applications to send and
receive strongly typed data objects (e.g., a receive call on an HTTP
Message Carrier should return an object representing the HTTP
response, with pre-parsed status code, headers, and any message body,
rather than returning a byte array that the application has to parse
itself). This is backwards compatible with existing protocols and
APIs, since the wire format of messages does not change, but gives a
Post Sockets stack additional information to allow it to make better
use of modern transport services.
The Post Sockets approach is therefore to raise the semantic level of
the transport API: applications should send and receive messages in
the form of meaningful, strongly typed, protocol data. Parsing and
serialising such messages should be a re-usable function of the
protocol stack instance not the application. This is well-suited to
implementation in modern systems languages, such as Swift, Go, Rust,
or C++, but can also be implemented with some loss of type safety in
C.
4.3. Message Size Limitations
Ideally, Messages can be of infinite size. However, protocol stacks
and protocol stack implementations may impose their own limits on
message sizing; For example, SCTP [RFC4960] and TLS
[I-D.ietf-tls-tls13] impose record size limitations of 64kB and 16kB,
respectively. Message sizes may also be limited by the available
buffer at the receiver, since a Message must be fully assembled by
the transport layer before it can be passed on to the application
layer. Since not every transport protocol stack implements the
signaling necessary to negotiate or expose message size limitations,
these may need to be defined out of band, and are probably best
exposed through the Configuration.
A truly infinite message service - e.g. large file transfer where
both endpoints have committed persistent storage to the message - is
probably best realized as a layer above Post Sockets, and may be
added as a new type of Message Carrier to a future revision of this
document.
4.4. Back-pressure
Regardless of how asynchronous reception is implemented, it is
important for an application to be able to apply receiver back-
pressure, to allow the protocol stack to perform receiver flow
control. Depending on how asynchronous I/O works in the platform,
this could be implemented by having a maximum number of concurrent
receive callbacks, or by bounding the maximum number of outstanding,
unread bytes at any given time, for example.
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Internet-Draft Post Sockets October 20174.5. Associations, Transients, Racing, and Rendezvous
As the network has evolved, even the simple act of establishing a
connection has become increasingly complex. Clients now regularly
race multiple connections, for example over IPv4 and IPv6, to
determine which protocol to use. The choice of outgoing interface
has also become more important, with differential reachability and
performance from multiple interfaces. Name resolution can also give
different outcomes depending on the interface the query was issued
from. Finally, but often most significantly, NAT traversal, relay
discovery, and path state maintenance messages are an essential part
of connection establishment, especially for peer-to-peer
applications.
Post Sockets accordingly breaks communication establishment down into
multiple phases:
o Gathering Locals
The set of possible Locals is gathered. In the simple case, this
merely enumerates the local interfaces and protocols, and
allocates ephemeral source ports for transients. For example, a
system that has WiFi and Ethernet and supports IPv4 and IPv6 might
gather four candidate locals (IPv4 on Ethernet, IPv6 on Ethernet,
IPv4 on WiFi, and IPv6 on WiFi) that can form the source for a
transient.
If NAT traversal is required, the process of gathering locals
becomes broadly equivalent to the ICE candidate gathering phase
[RFC5245]. The endpoint determines its server reflexive locals
(i.e., the translated address of a local, on the other side of a
NAT) and relayed locals (e.g., via a TURN server or other relay),
for each interface and network protocol. These are added to the
set of candidate locals for this association.
Gathering locals is primarily an endpoint local operation,
although it might involve exchanges with a STUN server to derive
server reflexive locals, or with a TURN server or other relay to
derive relayed locals. It does not involve communication with the
remote.
o Resolving the Remote
The remote is typically a name that needs to be resolved into a
set of possible addresses that can be used for communication.
Resolving the remote is the process of recursively performing such
name lookups, until fully resolved, to return the set of
candidates for the remote of this association.
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How this is done will depend on the type of the Remote, and can
also be specific to each local. A common case is when the Remote
is a DNS name, in which case it is resolved to give a set of IPv4
and IPv6 addresses representing that name. Some types of remote
might require more complex resolution. Resolving the remote for a
peer-to-peer connection might involve communication with a
rendezvous server, which in turn contacts the peer to gain consent
to communicate and retrieve its set of candidate locals, which are
returned and form the candidate remote addresses for contacting
that peer.
Resolving the remote is _not_ a local operation. It will involve
a directory service, and can require communication with the remote
to rendezvous and exchange peer addresses. This can expose some
or all of the candidate locals to the remote.
o Establishing Transients
The set of candidate locals and the set of candidate remotes are
paired, to derive a priority ordered set of Candidate Paths that
can potentially be used to establish a connection.
Then, communication is attempted over each candidate path, in
priority order. If there are multiple candidates with the same
priority, then transient establishment proceeds simultaneously and
uses the transient that wins the race to be established.
Otherwise, transients establishment is sequential, paced at a rate
that should not congest the network. Depending on the chosen
transport, this phase might involve racing TCP connections to a
server over IPv4 and IPv6 [RFC6555], or it could involve a STUN
exchange to establish peer-to-peer UDP connectivity [RFC5245], or
some other means.
o Confirming and Maintaining Transients
Once connectivity has been established, unused resources can be
released and the chosen path can be confirmed. This is primarily
required when establishing peer-to-peer connectivity, where
connections supporting relayed locals that were not required can
be closed, and where an associated signalling operation might be
needed to inform middleboxes and proxies of the chosen path.
Keep-alive messages may also be sent, as appropriate, to ensure
NAT and firewall state is maintained, so the transient remains
operational.
By encapsulating these four phases of communication establishment
into the PSI, Post Sockets aims to simplify application development.
It can provide reusable implementations of connection racing for TCP,
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to enable happy eyeballs, that will be automatically used by all TCP
clients, for example. With appropriate callbacks to drive the
rendezvous signalling as part of resolving the remote, we believe a
generic ICE implementation ought also to be possible. This procedure
can even be repeated fully or partially during a connection to enable
seamless hand-over and mobility within the network stack.
5. Acknowledgments
Many thanks to Laurent Chuat and Jason Lee at the Network Security
Group at ETH Zurich for contributions to the initial design of Post
Sockets. Thanks to Joe Hildebrand, Martin Thomson, and Michael Welzl
for their feedback, as well as the attendees of the Post Sockets
workshop in February 2017 in Zurich for the discussions, which have
improved the design described herein.
This work is partially supported by the European Commission under
Horizon 2020 grant agreement no. 688421 Measurement and Architecture
for a Middleboxed Internet (MAMI), and by the Swiss State Secretariat
for Education, Research, and Innovation under contract no. 15.0268.
This support does not imply endorsement.
6. References6.1. Normative References
[I-D.ietf-taps-transports]
Fairhurst, G., Trammell, B., and M. Kuehlewind, "Services
provided by IETF transport protocols and congestion
control mechanisms", draft-ietf-taps-transports-14 (work
in progress), December 2016.
6.2. Informative References
[I-D.ietf-quic-transport]
Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
and Secure Transport", draft-ietf-quic-transport-07 (work
in progress), October 2017.
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-21 (work in progress),
July 2017.
[I-D.iyengar-minion-protocol]
Jana, J., Cheshire, S., and J. Graessley, "Minion - Wire
Protocol", draft-iyengar-minion-protocol-02 (work in
progress), October 2013.
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